1. Field of the Invention
This invention relates to the control of cardiac ventricular tachyarrhythmias, including fibrillation, and more particularly to an improved method and apparatus employing an endocardial implementation technique for ease of implantation.
2. Description of the Prior Art
Ventricular fibrillation is a rapid, uncoordinated and ineffectual arrhythmia of the heart which results in death in a matter of a few minutes after onset unless treated effectively. The most effective therapy is application of an electrical shock of sufficient strength to depolarize most of the ventricular cells. Typically, for the hospitalized patient, the shock is applied by way of a pair of electrodes ("paddles") placed on the chest of the patient. An alternative effective therapy is implantation of an automatic defibrillator designed to detect the onset of fibrillation and to apply the corrective shock to an implanted electrode system.
Research to provide an automatic implantable standby defibrillator has been in progress for over fifteen years, yet the first implantation of such a device has only recently taken place. The very earliest attempts to directly defibrillate the exposed heart employed paddle-style electrodes which simulated the paddle electrodes used in external defibrillators. Because of the ease of implantation, efforts were quickly shifted towards the use of transvenous leads, and much effort was expended in attempting to provide a practical transvenous defibrillator. A suitable transvenous lead which met the requirements of ease of placement and minimization of defibrillation energy thresholds is described in the commonly assigned U.S. Pat. No. 4,355,646. Leads of the type described in this patent have been recently implanted with an automatic low energy cardioverter; however, endocardial leads have not been used chronically in patients with higher energy automatic implantable defibrillators.
Instead the lead systems employed clinically with an automatic implantable standby defibrillator for controlling ventricular fibrillation comprise an epicardial electrode applied to the lower part of the epicardium (the outer wall of the heart) and a second electrode inserted transvenously outside the heart in the superior vena cava (SVC) which is one of the large blood vessels. This lead system is the so-called "apical-SVC electrode system" which is shown in Heilman et al U.S. Pat. No. 4,030,509 and in the medical literature. Basically, it was found by Heilman et al and others working in the area that the energy required for defibrillation was lower using epicardial electrodes than could be obtained using the endocardial electrodes available to Heilman et al. In any event, it was also found necessary to employ the very large surface area electrodes that could be devised by using the epicardial approach because the energy required to defibrillate the average patient in actual clinical use remained relatively high and electrode-tissue current density dictated such large surface area electrodes.
The apical-SVC system contacts the left ventricle which comprises ninety percent of the heart tissue and is therefore efficient in distributing the defibrillating current throughout the ventricles, thereby minimizing to the extent possible the energy required to defibrillate. Unfortunately, major surgery is required to implant the apical electrode on the epicardium. For most candidate patients, such major surgery is not tolerable except as a last resort.
Turning to the stimulation technique, the automatic implantable defibrillators and cardioverters which have been implanted to date clinically employ a single pulse delivered between one set of electrodes on the lead systems previously described. It has been suggested in the past to defibrillate by use of sequential pulses applied to one or more sets of electrode pairs distributed on the epicardium of the heart or on the patient's chest for external use. For example, it has been suggested in the Tacker et al European Patent Application Ser. No. 0 095 726 that cardiac ventricular defibrillation or cardioversion may be treated by delivery of two sequential current pulses to separate pairs of electrodes orthogonally placed around the outside of the heart in the epicardial-pericardial space. It is claimed that this system provides a dramatic reduction in the voltage, current and energy required to defibrillate when compared to the previously described electrode systems. However, the system described by Tacker et al employs epicardial electrodes which must be precisely spaced about the epicardium and fixed in same manner to prevent their subsequent dislocation. Although Tacker et al propose a self-fixing electrode system applied through a limited thoracotomy, it is an open question whether such a system can be realized that will be reliable enough for chronic implantation. Instead, it is expected that a major thoracotomy may be required in order to adequately secure the electrodes in position.
Tacker et al further criticize any use of endocardial lead systems because of perceived problems of fibrosis, blood clots and embolus formations, and the associated risk of stroke and infarction. The system provided by Tacket et al therefore contemplates an entirely epicardial approach.